A selective catalytic reduction NOx catalyst (hereinafter simply referred to as “NOx catalyst” as well) is known, wherein NOx, which is contained in the exhaust gas discharged from an internal combustion engine, is purified by using ammonia as a reducing agent. For example, when NOx is purified by supplying ammonia previously stored in a tank to the NOx catalyst, it is necessary for a user to supplement ammonia to the tank.
In relation thereto, a technique is known, wherein ammonia is produced from water and nitrogen contained in the air (see, for example, Patent Document 1). When this technique is used, then it is unnecessary for the user to supplement ammonia, but it is necessary to supplement water in order to produce ammonia.